The thickness of a thin film of a conductor or a superconductor is one of the most important physical quantities to control in growing a thin film of such a conductor or superconductor material. In the case of a superconductor thin film, the magnitude of the effective surface resistance measured in a microwave band varies according to the thickness of the grown superconductor thin film. That is the reason why the thickness of a thin film must be known in order to measure the intrinsic surface resistance of a manufactured superconductor thin film. The magnitude of the critical current also varies according to the thickness of a superconductor thin film. In the case of a superconductor thin film manufactured for a Super-Conducting Quantum Interference Device (SQUID), variation in the critical current of a Josephson junction according to thickness greatly influences the performance of the manufactured SQUID. In the case of a conductor, there is a need to precisely measure the thickness of a conductor thin film from the standpoint of the fact that the resistance of a grown conductor thin film varies according to the thickness thereof. The effective surface resistance, a parameter for microwave applications, also varies according to the thickness of a conductor thin film when microwaves are applied to the conductor.
For reference, when the thickness of a conductor thin film or a superconductor thin film is three times as great as the penetration depth of electromagnetic waves (henceforth referred to as ‘the penetration depth’) or greater, the effective surface resistance of the thin film has almost the same value as the intrinsic surface resistance thereof. Such an electromagnetic wave penetration depth is given as a function of temperature and frequency in the case of a conductor, and as a function of temperature in the case of a superconductor.
For the measurement of the thickness of a thin film, measurement using Transmission Electron Microscopy (TEM) or Atomic Force Microscope (AFM), measurement using a Scanning Electron Microscope (SEM), measurement using an α-step profilometer, measurement using ellipsometry, etc. have been used. Of these measurement methods, only measurement using ellipsometry is a non-invasive measurement method. It is well known that, if the thickness of a thin film is measured using other methods, the original form of the thin film is deformed. Meanwhile, the measurement using ellipsometry is efficient in measuring the thickness of a insulator thin film or a semiconductor thin film in a non-invasive manner, but is inefficient in that, in the case of a conductor thin film, even if the thickness of a conductor thin film is only several nm or above, the thickness cannot be measured. The reason for this is that the penetration depth of electromagnetic waves into a conductor has a very small value of about several nm in a frequency band of several hundreds of THz, which is the frequency band of light used in ellipsometry.